Fuel cells are devices characterised by the possibility of converting the chemical energy of combination of a fuel and an oxidant directly into electrical energy. For this reason, fuel cells are not subject to the known limitation of Carnot's cycle, based on an intermediate thermal stage, and are therefore characterised by a high energy efficiency.
Among the several types under development at present, ion-exchange membrane fuel cells are considered of peculiar interest for automotive and for medium-small size residential generation applications, due to the compactness, the relative operating simplicity and the absence of important decay phenomena in the construction materials.
The oxidant normally employed in ion-exchange membrane fuel cells is air, at near-atmospheric pressure or under a pressure not exceeding 3-4 bar. The fuel, fed at about the same pressure as air, may consist of a gas containing hydrogen together with other components such as carbon dioxide and nitrogen, usually obtained by conversion of hydrocarbons or alcohols in reactors known as modifiers, among which steam reformers and autothermal reformers are the most common. Such type of gas is affected by two important drawbacks, namely the presence of residual traces of carbon monoxide, which is in fact an unavoidable intermediate compound in the conversion of hydrocarbons or alcohols into hydrogen, and the need of purging remarkable quantities of gas from the fuel cell to prevent an intolerable build-up of carbon monoxide and nitrogen in its interior. Carbon monoxide is capable, even as traces, to block the conventional platinum catalysts used in fuel cell electrodes forcing the use of special alloys, which are more expensive and whose reliability is still to be proved. Considerable purge flow-rates pose the problem of the utilisation of the hydrogen contained therein, for instance as fuel of the steam reforming reactor in automotive applications or as fuel for the generation of thermal energy in residential-type applications, in both cases with sensible complications of the overall system.
As an alternative, fuel cells may be fed with pure hydrogen, obtainable in particular cases by electrolysis or more generally from the same gas mixtures produced in steam reforming or autothermal reforming reactors through a passage across suitable separation units, based for instance on metal or polymer selective membranes or on absorbing materials such as certain types of molecular sieves. The fuel cell feed on pure hydrogen allows using the conventional platinum catalysts whose working reliability over an extended period of time is widely demonstrated and in principle to operate with unit stoichiometry flow-rates, in other words with flow-rates exactly corresponding to the amount of hydrogen consumed by the output current. Nevertheless this operating mode, known to those skilled in the art as “dead-end mode”, introduces at least two kinds of problems. The former problem is associated to the capacity of the nitrogen contained in the air on the fuel cell cathode compartment to diffuse across the ion-exchange membrane, progressively building up in the stagnant hydrogen present on the anode side, particularly near the bottom: the consequent hydrogen dilution determines a preferential distribution of the output current in those zones with a less relevant presence of nitrogen, with a consequent performance decay and a possible lifetime decrease. The latter problem is caused by the presence of liquid water formed by condensation of the water vapour transported by diffusion from the cathode compartment, similarly to what occurs with nitrogen. The progressive liquid water build-up leads to flooding phenomena of the porous anode catalyst structure, with an additional performance decay which sums up to the one induced by the accumulation of nitrogen. In order to obviate to this inconvenience, it was proposed to carry out periodical purges, allowing to extract nitrogen and water accumulated during the operating time: by appropriately regulating the purge frequency, it is possible to prevent this performance decay. This method, however, is effective with fuel cells operating under pressure, since only in this situation the decompression caused by the purging permits a deep renewal of the anodic gas: the decompression taking place in this way nevertheless determines an abnormal mechanical solicitation of the delicate ion-exchange membrane whose lifetime may thus be shortened.
As an alternative, it is possible to operate the fuel cell with an above-stoichiometric hydrogen flow-rate: the discharge of excess hydrogen allows to continuously withdraw nitrogen and water diffusing across the ion-exchange membrane before build-ups hampering the performances occur. The excess of hydrogen must however be exploited, with the consequent complications of the overall system, as already mentioned for the case of feeding with gas mixtures produced in modifiers.
The best prior art solution to the problems of nitrogen and water build-up perhaps consists of the external recycle of exhaust hydrogen through a mechanical pump: by suitably adjusting the recycle flow-rate, it is possible to establish a hydrogen flow in the anode compartment sufficient to keep both the nitrogen concentration and the water accumulation at low levels. A small amount of gas is withdrawn from the circuit to prevent the latter to be excessively enriched in nitrogen and water and to restore the conditions for the performance decay. However, the success of the method is based on the assumption that the overall hydrogen flow-rate consisting of the feed and the recycle be apportioned in a substantially uniform fashion over the multiplicity of fuel cells which must be assembled to obtained the high voltages normally required by the user appliances: since i the passage sections for the feeding of gases to the individual fuel cells have a random distribution around the design value, due to the constructive tolerances and of the inevitable, albeit marginal, imperfections in the assembly of the various components, the hydrogen flow-rate may be lower in some cells with respect to the average value, which makes the extraction of the liquid water from the anode compartment more difficult. The individual fuel cells in which this negative situation takes place are consequently characterised by lower performances, which in extreme cases may even lead the whole stack to be put out of service. This problem, hard to counteract in a system consisting of a multiplicity of components, moreover adds up to the negative aspect of the energy consumption of the recirculation pump.
The latter point was taken into consideration in U.S. 2004/0142215, which represents the closest prior art to our invention. U.S. 2004/0142215 proposes to replace the external recirculation pump with at least one hydrogen-transfer electrochemical cell: this type of cell has a design equivalent to that of the fuel cell, and includes an ion-exchange membrane on whose faces two electrodes, anode and cathode, are applied, both containing a catalyst suitable for hydrogen ionisation to protons and for proton recombination to hydrogen. During operation, the hydrogen-transfer cell anode compartment is fed with the hydrogen exhaust of the fuel cell. The anode provides to the ionisation of hydrogen to protons which migrate across the membrane and are recombined on the opposite side at the cathode, producing pure hydrogen. This product hydrogen is added to the main feed, giving thus rise to the same kind of external recycling which would be obtained by installing a mechanical pump: the ratio between number of hydrogen-transfer cells and number of fuel cells defines the recirculation rate. The hydrogen-transfer cells, installed as a separate module distinct from the fuel cell one or laminated with the fuel cells in a single assembly, may be fed with an external electrical source or otherwise with a portion of the fuel cell electric output. The device of U.S. 2004/0142215 has the merit of eliminating a delicate component such as the recirculation pump with its rotating parts, while retaining the above seen inconveniences of possible malfunctioning of some individual cells presenting reduced passage sections and lower internal hydrogen flow-rates.